I have built this amp based off the attached schematic. When the left RCA cable is unplugged the left side buzzes/motor boats. No effect on the right channel. I generally attribute motorboating to a bad electrolytic cap in the power supply but I have checked all the PS caps and they are good. As long as the RCAs are plugged in the amp sounds great. Any ideas?
Maybe a solder joint? A preamp I built this week had a problem with one channel. It looked exactly like the other channel. The problem turned out to be a bad solder joint on a ground wire. The 14 gauge stranded wire was very well attached physically by the solder joint, yet the connection measured in the meg ohms. The weirdest thing I've ever seen. I eventually found it using the ohm meter and just checking everything comparing the two channels.
Motorboating low frequency feedback.
So whatever guess where that is coming from.
Like most assume it is just lead dress.
But can be transformer outputs to close to inputs.
Or as mentioned inputs and outputs
grounds mounted to chassis
so the chassis is big loop.
Or you have 2 power tubes sharing same resistor
with bypass cap.
So 33uf might not be enough power supply de coupling
for that stage.
Or have dedicated resistor for each power tube.
No inductor either so often helps.
Or slight degen for power tubes.
Not sure why stacking caps in series
for lower voltage stage.
Could just use same old 33u 600 volt
in previous stages.
Maybe cost, not sure
So whatever guess where that is coming from.
Like most assume it is just lead dress.
But can be transformer outputs to close to inputs.
Or as mentioned inputs and outputs
grounds mounted to chassis
so the chassis is big loop.
Or you have 2 power tubes sharing same resistor
with bypass cap.
So 33uf might not be enough power supply de coupling
for that stage.
Or have dedicated resistor for each power tube.
No inductor either so often helps.
Or slight degen for power tubes.
Not sure why stacking caps in series
for lower voltage stage.
Could just use same old 33u 600 volt
in previous stages.
Maybe cost, not sure
How are all those grounds in real life? For instance the two RCAs and then the volume control, input tube cathode resistor, etc? Is it a PCB or point to point?
I am trying to solve a similar issue on an amp I built in two small a box, with form over function, and I am suspecting the first stage lead dress and proximity to heater wiring.
Maybe a picture could be useful?
I am trying to solve a similar issue on an amp I built in two small a box, with form over function, and I am suspecting the first stage lead dress and proximity to heater wiring.
Maybe a picture could be useful?
If you have motorboating, it's an interaction between the low frequency time constants in the power supply filtering and the amplifier's audio response, set by coupling/decoupling capacitances, not stray capacitances. This assumes there isn't a gross wiring error (incorrect resistor etc) in one channel. Compare voltages between channels; they should be pretty much the same. The old cure for motorboating was to reduce the size of the coupling capacitor (your 0.1uF 600V), but I'd be far more suspicious of your shared cathode resistor and capacitance at the output stage. Use individual resistances of twice the value and reduce the capacitance to 47uF for each.
It is built in a wood chassis. Inputs, outputs, and transformer Are well separated. I built this point to point with no volume control- instead putting a 100k grid leak resistor across the inputs. All audio stage grounds are star grounded to one point. All power stage grounds are grounded to a different point, with both separate star ground points then tied to the earth ground at the plug.
No volume control???
This KIT/schematic has no grid leak resistor in first stage (IMO whoever committed this is also capable of other mistakes
), only the volume potentiometer acts as grid leak.... if the contact of the slider is OK.
If you omitted the potentiometer, the first stage is in undefined operating point, so motorboating even occurs.
This KIT/schematic has no grid leak resistor in first stage (IMO whoever committed this is also capable of other mistakes
), only the volume potentiometer acts as grid leak.... if the contact of the slider is OK.If you omitted the potentiometer, the first stage is in undefined operating point, so motorboating even occurs.
I checked the resistance of all connections, checked the resistors, and replaced both the PS and coupling caps. Motorboating on both channels if the RCA is unplugged now -> meaning if I I remove the left RCA the left channel chirps, and if I remove the right RCA the right channel chirps. The channel with the RCA plugged in is fine and plays normal. If both RCAs are unplugged both sides chirp and if both stay plugged in both channels sound amazing.